The present disclosure incorporates the entire disclosure of PCT Application Publication No. WO 2016/200266 A1 for all purposes, including specifically the entire disclosure relating to the disclosed embodiments of the apparatus.
The present disclosure relates to monitoring of a wellbore, and in particular to monitoring and determining properties of leakages and/or detecting leaked material from such barriers, such as fluid which may leak from a formation through a barrier in a permanently and/or temporarily abandoned well.
When wells are to be abandoned or plugged, barriers are typically installed deep in the wellbore in the Earth's subsurface to prevent for example fluids from propagating up the wellbore and out of the well at the surface. The barriers may be provided with a view of staying there permanently, or temporarily for a period of time, until the wellbore is put to use later on. Typically, the barriers are designed to be long-term solutions for example to seal the well for a period of months, years or permanently. The barriers are required to seal the wellbore to withstand the pressure of fluids below the barrier and prevent fluids from travelling up to the surface via the wellbore. A particular application for such barriers is in wells that have been used in the exploration and production of oil and gas, or water and/or gas injection wells which may for example be applied to facilitate such exploration and production activities. Barriers of similar sort are used in wells in other industries, such as in wells which may be used to store radioactive waste or the like within the Earth's crust, and may also be applied in gas storage wells, CO2 storage wells or geothermal wells.
Requirements and procedures for plugging and abandoning wells are regulated by standards predicted by governmental authorities in respective countries. Standards for plugging and abandoning wells in Norway are set out in the standard Norsok D-010. Barriers established in order to seal off reservoir sections in wells may be separate or combined, and have to be tested accordingly. Such barriers are typically formed from cement, by inserting cement slurry into the wellbore and leaving it to set, although other materials can be used such as SANDABAND™ and/or molten alloy sealing materials, and mechanical plugging devices can also be used to provide barriers.
The requirements also make it necessary for each of the barriers to provide a so-called full bore seal. In order to do so, permanently and temporarily abandoned wells have commonly been provided with barriers which are installed in open-hole, uncased sections of the wellbore, or in sections where casing has been removed, e.g., by milling or pulling out sections of casing. Increasingly however, it has been of interest to permanently abandon wells in cased sections without removing casing sections, for example to save costs and/or to facilitate re-use of the abandoned wellbore several years later. The barriers must then typically seal the wellbore, the formation annulus between the outside of the casing and the formation, and any casing annulus between two casings.
A large number of wells throughout the world are candidates for different types of abandonment.
Although a wellbore may have barriers installed to sufficient standards, leakages are sometimes experienced after a period of time, e.g. shortly after installation or after a period of several months or years. If the leakages are substantial, remedial work may be required.
In cased wellbores, a particular difficulty is that there are multiple potential leakage paths which arise, such as along the outer surface of the casing, between the casing and the formation and/or between two adjacent casings. If the seal between the casing and the formation is leaky, fluids from below the barrier may migrate upward along the outer surface of the casing to the surface. This may cause undesired contamination into the environment, e.g. into the sea in the case of offshore wells. There may also be a risk of leaking fluids entering groundwater reservoirs, and causing undesired contamination of water to be supplied to consumers.
While keeping the casing in the wellbore can be beneficial on one hand, the leakage behaviour may be more complicated due to the presence of casings, and remedial work may be more difficult.
In light of this above there is a need for better understanding the leakage behaviour of barriers in abandoned wells, and to reliably detect leaking barriers, or components thereof, in order that remedial actions can be taken.